Colony stimulating factor 1 receptor (referred to herein as CSF1R; also referred to in the art as FMS, FIM2, C-FMS, M-CSF receptor, and CD115) is a single-pass transmembrane receptor with an N-terminal extracellular domain (ECD) and a C-terminal intracellular domain with tyrosine kinase activity. Ligand binding of CSF1 or the interleukin 34 ligand (referred to herein as IL-34; Lin et al., Science 320: 807-11 (2008)) to CSF1R leads to receptor dimerization, upregulation of CSF1R protein tyrosine kinase activity, phosphorylation of CSF1R tyrosine residues, and downstream signaling events. Both CSF1 and IL-34 stimulate monocyte survival, proliferation, and differentiation into macrophages, as well as other monocytic cell lineages such as osteoclasts, dendritic cells, and microglia.
Many tumor cells have been found to secrete CSF1, which activates monocyte/macrophage cells through CSF1R. The level of CSF1 in tumors has been shown to correlate with the level of tumor-associated macrophages (TAMs) in the tumor. Higher levels of TAMs have been found to correlate with poorer patient prognoses. In addition, CSF1 has been found to promote tumor growth and progression to metastasis in, for example, human breast cancer xenografts in mice. See, e.g., Paulus et al., Cancer Res. 66: 4349-56 (2006). Further, CSF1R plays a role in osteolytic bone destruction in bone metastasis. See, e.g., Ohno et al., Mol. Cancer Ther. 5: 2634-43 (2006).
CSF1 and its receptor have also been found to be involved in various inflammatory and autoimmune diseases. See, e.g., Hamilton, Nat. Rev. 8: 533-44 (2008). For example, synovial endothelial cells from joints afflicted with rheumatoid arthritis have been found to produce CSF1, suggesting a role for CSF1 and its receptor in the disease. Blocking CSF1R activity with an antibody results in positive clinical effects in mouse models of arthritis, including a reduction in the destruction of bone and cartilage and a reduction in macrophage numbers. See, e.g., Kitaura et al., J. Clin. Invest. 115: 3418-3427 (2005).
Mature differentiated myeloid lineage cells such as macrophages, microglial cells, and osteoclasts contribute to pathology of various diseases such as rheumatoid arthritis, multiple sclerosis and diseases of bone loss. Differentiated myeloid lineage cells are derived from peripheral blood monocyte intermediates. CSF1R stimulation contributes to development of monocytes from bone marrow precursors, to monocyte proliferation and survival, and to differentiation of peripheral blood monocytes into differentiated myeloid lineage cells such as macrophages, microglial cells, and osteoclasts. CSF1R stimulation thus contributes to proliferation, survival, activation, and maturation of differentiated myeloid lineage cells, and in the pathologic setting, CSF1R stimulation contributes to the ability of differentiated myeloid lineage cells to mediate disease pathology.